In the field of lighting, present efforts aim for reducing the power consumption in particular of residential and commercial lighting applications. Nowadays, lamps or light sources are being employed for the replacement of common incandescent or halogen lamps, which include one or more light emitting diodes (LEDs). LEDs feature a dramatically decreased power consumption in comparison to incandescent lamps with the same luminous flux and in addition provide a substantially increased lifetime. LEDs thus are very promising for new generation light sources.
For retrofit applications however, i.e. as a replacement for incandescent or halogen lamps, LEDs typically cannot be used directly with common types of installed power supplies, but due to its exponential voltage-current behavior, require a dedicated driver circuit. The LED driver circuit usually adapts the voltage for the LED to the requisite level and also keeps the delivered current constant. A most simple “driver circuit” comprises a resistive element in series with the LED(s).
A particular problem may arise from the fact that the reduced power consumption of LEDs results in an accordingly reduced operating current. In particular when using the LED lamp with a power supply having a phase-cut dimming unit, such as a leading-edge or trailing-edge dimming unit, the power supply may have a minimum load requirement, which may not be met by the LED lamp. Here, the reduced current may result in an unintended behavior of the dimmer/lamp combination, which may e.g. result in visible flicker.
For example, since a trailing-edge (TE) type of dimmer typically is MOSFET-based and comprises an internal supply circuit which powers the timing and zero-crossing detection circuit, the “under-load” may result in that the internal supply cannot provide enough operating power to the timing circuit, causing problems with the zero-crossing detection of the sinusoidal mains voltage. In a leading-edge (LE) type of dimmer, a TRIAC or two anti-parallel connected thyristors are used, where the current typically needs to be high enough to maintain the TRIAC in a conductive state, i.e. above a holding current, so that the operation of a LED lamp with this type of power supply or dimmer may result in an “unintended” or untimely disconnect of the TRIAC(s).
An object of the present invention therefore is to provide a circuit arrangement, which allows operating at least one low-power lighting unit with a phase-cut operating voltage, so that it is possible to operate said lighting unit with a variety of types of power supplies while maintaining high quality light output. A further object is to provide a cost-efficient circuit arrangement, which allows to be used for mass market applications.